In the families of patients with Juvenile Diabetes Mellitus (JDM) we have found the disease to be inherited in association with HLA-D (formerly called LD or lymphocyte-determined) antigens - Dissociation between the HLA-D antigens and the rest of the haplotype occurred because of a very high rate of intra-HLA recombination. Intra-HLArecombinations are significantly more frequent in families with JDM than in the normal population (p equals 2X10 to the minus 6 power). We want to further investigate the nature of the association of JDM with HLA as a means to characterize the mode of inheritance of JDM and predict risks to individuals in affected sibships. For this, we shall study the segregation of JDM with different genes of the HLA linkage group and other genetic markers in a large number of families. As controls, families with adult-onset diabetes will be similarly tested.